A scroll type compressor with a variable displacement mechanism is well known in the art. For example, U.S. Pat. No. 4,744,733 shows such a compressor.
In U.S. Pat. No. 4,744,733, the variable displacement mechanism comprises both first and second valve elements. The second valve element controls the motion of the first valve element. The first valve element comprises a cylinder and a piston slidably disposed within the cylinder. A top of the piston of the first valve element receives varied pressure of a compressed fluid in the discharge chamber by virtue of the second valve element. The cylinder of the first valve element has both first and second openings. The first opening is formed at an intermediate pressure chamber side. The second opening is formed at a communication channel side. The bottom of the piston of the first valve element blocks the first opening of the cylinder of the first valve element when the top of the piston of the first valve element receives the pressure of the compressed fluid in the discharge chamber.
In this variable displacement mechanism, there are mainly two problems as described in the following. One problem is in firmly blocking the first opening of the cylinder by the bottom of the piston, because of being small pressure difference between the discharge chamber pressure and the intermediate chamber pressure.
Furthermore, U.S. Pat. No. 4,505,651 shows another variable displacement mechanism. This variable displacement mechanism comprises one valve element as an electromagnetic valve directly controlling the opening and closing of a communicating hole formed at a circular end plate of a fixed scroll. This mechanism cannot obtain a firm block of the communicating hole which communicates between a suction chamber and an intermediate chamber, because of directly controlling of the opening and the closing of the communicating hole.
Another problem is a fluttering of the piston when the piston begins opening. This fluttering of the piston happens by a quick change of pressure which pushes the bottom of the piston.